Method for base station transmitting downlink control channel in wireless communication system and apparatus for same

ABSTRACT

In the present invention, a method for a base station transmitting a downlink channel is disclosed. More particularly, the method comprises the steps of: dividing each of one or more resources blocks, which are allocated for the downlink control channel, into a predetermined number of subsets; deciding the number of subsets that comprise a resource allocation basic unit for the downlink control channel, based on a start symbol and/or an end symbol of the downlink control channel; mapping a transmission resource on the downlink control channel as the resource allocation basic unit comprising the predetermined number of subsets; and transmitting the downlink control channel by using the transmission resource that is mapped.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method and apparatus for transmitting a downlinkcontrol channel at a base station in a wireless communication system.

BACKGROUND ART

A brief description will be given of a 3rd Generation PartnershipProject Long Term Evolution (3GPP LTE) system as an example of awireless communication system to which the present invention can beapplied.

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an exemplary wirelesscommunication system. The E-UMTS system is an evolution of the legacyUMTS system and the 3GPP is working on the basics of E-UMTSstandardization. E-UMTS is also called an LTE system. For details of thetechnical specifications of UMTS and E-UMTS, refer to Release 7 andRelease 8 of “3rd Generation Partnership Project; TechnicalSpecification Group Radio Access Network”, respectively.

Referring to FIG. 1, the E-UMTS system includes a User Equipment (UE),an evolved Node B (eNode B or eNB), and an Access Gateway (AG) which islocated at an end of an Evolved UMTS Terrestrial Radio Access Network(E-UTRAN) and connected to an external network. The eNB may transmitmultiple data streams simultaneously, for broadcast service, multicastservice, and/or unicast service.

A single eNB manages one or more cells. A cell is set to operate in oneof the bandwidths of 1.44, 3, 5, 10, 15 and 20 Mhz and provides Downlink(DL) or Uplink (UL) transmission service to a plurality of UEs in thebandwidth. Different cells may be configured so as to provide differentbandwidths. An eNB controls data transmission and reception to and froma plurality of UEs. Regarding DL data, the eNB notifies a particular UEof a time-frequency area in which the DL data is supposed to betransmitted, a coding scheme, a data size, Hybrid Automatic RepeatreQuest (HARQ) information, etc. by transmitting DL schedulinginformation to the UE. Regarding UL data, the eNB notifies a particularUE of a time-frequency area in which the UE can transmit data, a codingscheme, a data size, HARQ information, etc. by transmitting ULscheduling information to the UE. An interface for transmitting usertraffic or control traffic may be defined between eNBs. A Core Network(CN) may include an AG and a network node for user registration of UEs.The AG manages the mobility of UEs on a Tracking Area (TA) basis. A TAincludes a plurality of cells.

While the development stage of wireless communication technology hasreached LTE based on Wideband Code Division Multiple Access (WCDMA), thedemands and expectation of users and service providers are increasing.Considering that other radio access technologies are under development,a new technological evolution is required to achieve futurecompetitiveness. Specifically, cost reduction per bit, increased serviceavailability, flexible use of frequency bands, a simplified structure,an open interface, appropriate power consumption of UEs, etc. arerequired.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ona method and apparatus for transmitting a downlink control channel at abase station in a wireless communication system.

Technical Solution

In an aspect of the present invention, a method for transmitting adownlink control channel at a base station in a wireless communicationsystem includes dividing each of one or more resource blocks allocatedto the downlink control channel into a predetermined number of subsets,determining a number of subsets that form a basic resource allocationunit for the downlink control channel, based on at least one of startingand ending symbols of the downlink control channel, mapping transmissionresources to the downlink control channel, using a basic resourceallocating unit configured with the determined number of subsets, andtransmitting the downlink control channel in the mapped transmissionresources.

The transmission resources may be an aggregate of one or more basicresource allocation units. In addition, the transmission resources maybe located in a data region of a subframe.

If the index of the starting symbol of the downlink control channel issmaller than a first value, the number of subsets may be determined tobe a value equal to or larger than 1 and smaller than the predeterminednumber. If the index of the starting symbol of the downlink controlchannel is equal to or larger than a first value, the number of subsetsmay be determined to be the predetermined number.

If the index of the ending symbol of the downlink control channel isequal to or larger than a second value, the number of subsets may bedetermined to be a value equal to or larger than 1 and smaller than thepredetermined number. If the index of the ending symbol of the downlinkcontrol channel is smaller than a second value, the number of subsetsmay be determined to be the predetermined number.

In another aspect of the present invention, a method for receiving adownlink control channel at a user equipment in a wireless communicationsystem includes configuring a basic resource allocation unit for thedownlink control channel, using one or more resource blocks allocated tothe downlink control channel, and receiving the downlink control channelby monitoring a search space on the basic resource allocation unit basisaccording to an aggregation level. Each of the one or more resourceblocks is divided into a predetermined number of subsets and a number ofsubsets that form the basic resource allocation unit is determined basedon at least one of starting and ending symbols of the downlink controlchannel.

Resources in which the downlink control channel is received may be anaggregate of one or more basic resource allocation units. The resourcesin which the downlink control channel may be located in a data region ofa subframe.

If the index of the starting symbol of the downlink control channel issmaller than a first value, the number of subsets may be equal to orlarger than 1 and smaller than the predetermined number. If the index ofthe starting symbol of the downlink control channel is equal to orlarger than a first value, the number of subsets may be thepredetermined number.

If the index of the ending symbol of the downlink control channel isequal to or larger than a second value, the number of subsets may beequal to or larger than 1 and smaller than the predetermined number. Ifthe index of the ending symbol of the downlink control channel issmaller than a second value, the number of subsets may be thepredetermined number.

Advantageous Effects

According to embodiments of the present invention, a base station canefficiently transmit a downlink control channel in a wirelesscommunication system.

It will be appreciated by persons skilled in the art that that theeffects that can be achieved through the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a configuration of an Evolved Universal MobileTelecommunications System (E-UMTS) network as an example of a wirelesscommunication system;

FIG. 2 illustrates a control-plane protocol stack and a user-planeprotocol stack in a radio interface protocol architecture conforming toa 3rd Generation Partnership Project (3GPP) radio access networkstandard between a User Equipment (UE) and an Evolved UMTS TerrestrialRadio Access Network (E-UTRAN);

FIG. 3 illustrates physical channels and a general signal transmissionmethod using the physical channels in a 3GPP system;

FIG. 4 illustrates a configuration of a Multiple Input Multiple Output(MIMO) communication system;

FIG. 5 illustrates a structure of a downlink radio frame in a Long TermEvolution (LTE) system;

FIG. 6 illustrates resources units used for configuring a downlinkcontrol channel in the LTE system;

FIG. 7 illustrates a structure of an uplink radio frame in the LTEsystem;

FIG. 8 illustrates a configuration of a multi-node system as afuture-generation communication system;

FIG. 9 illustrates an example of an Enhanced Physical Downlink ControlChannel (E-PDCCH) and a Physical Downlink Shared Channel (PDSCH)scheduled by the E-PDCCH;

FIG. 10 illustrates an example of defining a basic unit for configuringan E-PDCCH as two sub-Physical Radio Resource Blocks (sub-PRBs)according to an embodiment of the present invention;

FIG. 11 illustrates an example of defining a basic aggregation unit foran E-PDCCH according to another embodiment of the present invention; and

FIG. 12 is a block diagram of a communication apparatus according to anembodiment of the present invention.

BEST MODE

The configuration, operation, and other features of the presentinvention will readily be understood with embodiments of the presentinvention described with reference to the attached drawings. Embodimentsof the present invention as set forth herein are examples in which thetechnical features of the present invention are applied to a 3rdGeneration Partnership Project (3GPP) system.

While embodiments of the present invention are described in the contextof Long Term Evolution (LTE) and LTE-Advanced (LTE-A) systems, they arepurely exemplary. Therefore, the embodiments of the present inventionare applicable to any other communication system as long as the abovedefinitions are valid for the communication system. In addition, whilethe embodiments of the present invention are described in the context ofFrequency Division Duplexing (FDD), they are also readily applicable toHalf-FDD (H-FDD) or Time Division Duplexing (TDD) with somemodifications.

FIG. 2 illustrates control-plane and user-plane protocol stacks in aradio interface protocol architecture conforming to a 3GPP wirelessaccess network standard between a User Equipment (UE) and an EvolvedUMTS Terrestrial Radio Access Network (E-UTRAN). The control plane is apath in which the UE and the E-UTRAN transmit control messages to managecalls, and the user plane is a path in which data generated from anapplication layer, for example, voice data or Internet packet data istransmitted.

A PHYsical (PHY) layer at Layer 1 (L1) provides information transferservice to its higher layer, a Medium Access Control (MAC) layer. ThePHY layer is connected to the MAC layer via transport channels. Thetransport channels deliver data between the MAC layer and the PHY layer.Data is transmitted on physical channels between the PHY layers of atransmitter and a receiver. The physical channels use time and frequencyas radio resources. Specifically, the physical channels are modulated inOrthogonal Frequency Division Multiple Access (OFDMA) for Downlink (DL)and in Single Carrier Frequency Division Multiple Access (SC-FDMA) forUplink (UL).

The MAC layer at Layer 2 (L2) provides service to its higher layer, aRadio Link Control (RLC) layer via logical channels. The RLC layer at L2supports reliable data transmission. RLC functionality may beimplemented in a function block of the MAC layer. A Packet DataConvergence Protocol (PDCP) layer at L2 performs header compression toreduce the amount of unnecessary control information and thusefficiently transmit Internet Protocol (IP) packets such as IP version 4(IPv4) or IP version 6 (IPv6) packets via an air interface having anarrow bandwidth.

A Radio Resource Control (RRC) layer at the lowest part of Layer 3 (orL3) is defined only on the control plane. The RRC layer controls logicalchannels, transport channels, and physical channels in relation toconfiguration, reconfiguration, and release of radio bearers. A radiobearer refers to a service provided at L2, for data transmission betweenthe UE and the E-UTRAN. For this purpose, the RRC layers of the UE andthe E-UTRAN exchange RRC messages with each other. If an RRC connectionis established between the UE and the E-UTRAN, the UE is in RRCConnected mode and otherwise, the UE is in RRC Idle mode. A Non-AccessStratum (NAS) layer above the RRC layer performs functions includingsession management and mobility management.

A cell managed by an evolved Node B (eNB or eNode B) is set to one ofbandwidths of 1.4, 3, 5, 10, 15, and 20 Mhz and provides a DL or ULservice to a plurality of UEs. Different cells may be set to differentbandwidths.

DL transport channels used to deliver data from the E-UTRAN to UEsinclude a Broadcast Channel (BCH) carrying system information, a PagingChannel (PCH) carrying a paging message, and a Shared Channel (SCH)carrying user traffic or a control message. DL multicast traffic orcontrol messages or DL broadcast traffic or control messages may betransmitted on a DL SCH or a separately defined DL Multicast Channel(MCH). UL transport channels used to deliver data from a UE to theE-UTRAN include a Random Access Channel (RACH) carrying an initialcontrol message and a UL SCH carrying user traffic or a control message.Logical channels that are defined above transport channels and mapped tothe transport channels include a Broadcast Control Channel (BCCH), aPaging Control Channel (PCCH), a Common Control Channel (CCCH), aMulticast Control Channel (MCCH), a Multicast Traffic Channel (MTCH),etc.

FIG. 3 illustrates physical channels and a general method fortransmitting signals on the physical channels in the 3GPP system.

Referring to FIG. 3, when a UE is powered on or enters a new cell, theUE performs initial cell search (S301). The initial cell search involvesacquisition of synchronization to an eNB. Specifically, the UEsynchronizes its timing to the eNB and acquires a cell Identifier (ID)and other information by receiving a Primary Synchronization Channel(P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.Then the UE may acquire information broadcast in the cell by receiving aPhysical Broadcast Channel (PBCH) from the eNB. During the initial cellsearch, the UE may monitor a DL channel state by receiving a DownLinkReference Signal (DL RS).

After the initial cell search, the UE may acquire detailed systeminformation by receiving a Physical Downlink Control Channel (PDCCH) andreceiving a Physical Downlink Shared Channel (PDSCH) based oninformation included in the PDCCH (S302).

If the UE initially accesses the eNB or has no radio resources forsignal transmission to the eNB, the UE may perform a random accessprocedure with the eNB (S303 to S306). In the random access procedure,the UE may transmit a predetermined sequence as a preamble on a PhysicalRandom Access Channel (PRACH) (S303 and S305) and may receive a responsemessage to the preamble on a PDCCH and a PDSCH associated with the PDCCH(S304 and S306). In the case of a contention-based RACH, the UE mayadditionally perform a contention resolution procedure.

After the above procedure, the UE may receive a PDCCH and/or a PDSCHfrom the eNB (S307) and transmit a Physical Uplink Shared Channel(PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB(S308), which is a general DL and UL signal transmission procedure.Particularly, the UE receives Downlink Control Information (DCI) on aPDCCH. Herein, the DCI includes control information such as resourceallocation information for the UE. Different DCI formats are definedaccording to different usages of DCI.

Control information that the UE transmits to the eNB on the UL orreceives from the eNB on the DL includes a DL/UL ACKnowledgment/NegativeACKnowledgment (ACK/NACK) signal, a Channel Quality Indicator (CQI), aPrecoding Matrix Index (PMI), a Rank Indicator (RI), etc. In the 3GPPLTE system, the UE may transmit control information such as a CQI, aPMI, an RI, etc. on a PUSCH and/or a PUCCH.

Now a description will be given of a Multiple Input Multiple Output(MIMO) system. MIMO can increase the transmission and receptionefficiency of data by using a plurality of Transmission (Tx) antennasand a plurality of Reception (Rx) antennas. That is, with the use ofmultiple antennas at a transmitter or a receiver, MIMO can increasecapacity and improve performance in a wireless communication system. Theterm “MIMO” is interchangeable with ‘multi-antenna’.

The MIMO technology does not depend on a single antenna path to receivea whole message. Rather, it completes the message by combining datafragments received through a plurality of antennas. MIMO can increasedata rate within a cell area of a predetermined size or extend systemcoverage at a given data rate. In addition, MIMO can find its use in awide range including mobile terminals, relays, etc. MIMO can overcome alimited transmission capacity encountered with the conventionalsingle-antenna technology in mobile communication.

FIG. 4 illustrates the configuration of a typical MIMO communicationsystem. Referring to FIG. 4, a transmitter has N_(T) Tx antennas and areceiver has N_(R) Rx antennas. The use of a plurality of antennas atboth the transmitter and the receiver increases a theoretical channeltransmission capacity, compared to the use of a plurality of antennas atonly one of the transmitter and the receiver. The channel transmissioncapacity increases in proportion to the number of antennas. Therefore,transmission rate and frequency efficiency are increased. Given amaximum transmission rate R_(o) that may be achieved with a singleantenna, the transmission rate may be increased, in theory, to theproduct of R_(o) and a transmission rate increase rate R_(i) in the caseof multiple antennas. R_(i) is the smaller value between N_(T) andN_(R).

R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was verified in themiddle 1990s, many techniques have been actively proposed to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards such asstandards for 3G mobile communications, future-generation Wireless LocalArea Network (WLAN), etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many aspects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system with N_(T) Tx antennas and N_(R) Rxantennas as illustrated in FIG. 4 will be described in detail throughmathematical modeling. Regarding a transmission signal, up to N_(T)pieces of information can be transmitted through the N_(T) Tx antennas,as expressed as the following vector.

$\begin{matrix}{s = \left\lbrack {s_{1},s_{2},\ldots \mspace{14mu},s_{N_{T}}} \right\rbrack^{T}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given as

$\begin{matrix}{\hat{s} = {\left\lbrack {{\hat{s}}_{1},{\hat{s}}_{2},\ldots \mspace{14mu},{\hat{s}}_{N_{T}}} \right\rbrack^{T} = \left\lbrack {{P_{1}s_{1}},{P_{2}s_{2}},\ldots \mspace{14mu},{P_{N_{T}}s_{N_{T}}}} \right\rbrack^{T}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

The transmission power-controlled transmission information vector S maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

N_(T) transmission signals x₁, x₂, . . . , x_(N) _(T) may be generatedby multiplying the transmission power-controlled information vector ŝ bya weight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These N_(T) transmission signals x₁,x₂, . . . , x_(N) _(T) are represented as a vector X, which may bedetermined by [Equation 5]. Herein, w_(ij) denotes a weight between aj^(th) piece of information and an i^(th) Tx antenna and W is referredto as a weight matrix or a precoding matrix.

                                 [Equation  5] $x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1\; N_{T}} \\w_{21} & w_{22} & \ldots & w_{2\; N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {\quad{{W\hat{s}} = {WPs}}}}}$

In general, the rank of a channel matrix is the maximum number ofdifferent pieces of information that can be transmitted on a givenchannel, in its physical meaning. Therefore, the rank of a channelmatrix is defined as the smaller between the number of independent rowsand the number of independent columns in the channel matrix. The rank ofthe channel matrix is not larger than the number of rows or columns ofthe channel matrix. The rank of a channel matrix H, rank(H) satisfiesthe following constraint.

rank(H)≦min(N _(T) ,N _(R))  [Equation 6]

A different piece of information transmitted in MIMO is referred to as‘transmission stream’ or shortly ‘stream’. The ‘stream’ may also becalled ‘layer’. It is thus concluded that the number of transmissionstreams is not larger than the rank of channels, i.e. the maximum numberof different pieces of transmittable information. Thus, the channelmatrix H is determined by

# of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 7]

“# of streams” denotes the number of streams. One thing to be notedherein is that one stream may be transmitted through one or moreantennas.

One or more streams may be mapped to a plurality of antennas in manyways. The stream-to-antenna mapping may be described as followsdepending on MIMO schemes. If one stream is transmitted through aplurality of antennas, this may be regarded as spatial diversity. When aplurality of streams are transmitted through a plurality of antennas,this may be spatial multiplexing. Needless to say, a hybrid scheme ofspatial diversity and spatial multiplexing in combination may becontemplated.

FIG. 5 illustrates exemplary control channels included in a controlregion of a subframe in a DL radio frame.

Referring to FIG. 5, a subframe includes 14 OFDM symbols. The first oneto three OFDM symbols of a subframe are used for a control region andthe other 13 to 11 OFDM symbols are used for a data region according toa subframe configuration. In FIG. 5, reference characters R1 to R4denote RSs or pilot signals for antenna 0 to antenna 3. RSs areallocated in a predetermined pattern in a subframe irrespective of thecontrol region and the data region. A control channel is allocated tonon-RS resources in the control region and a traffic channel is alsoallocated to non-RS resources in the data region. Control channelsallocated to the control region include a Physical Control FormatIndicator Channel (PCFICH), a Physical Hybrid-ARQ Indicator Channel(PHICH), a Physical Downlink Control Channel (PDCCH), etc.

The PCFICH is a physical control format indicator channel carryinginformation about the number of OFDM symbols used for PDCCHs in eachsubframe. The PCFICH is located in the first OFDM symbol of a subframeand configured with priority over the PHICH and the PDCCH. The PCFICHincludes 4 Resource Element Groups (REGs), each REG being distributed tothe control region based on a cell Identifier (ID). One REG includes 4Resource Elements (REs). An RE is a minimum physical resource defined byone subcarrier by one OFDM symbol. The PCFICH is set to 1 to 3 or 2 to 4according to a bandwidth. The PCFICH is modulated in Quadrature PhaseShift Keying (QPSK).

The PHICH is a physical Hybrid-Automatic Repeat and request (HARQ)indicator channel carrying an HARQ ACK/NACK for a UL transmission. Thatis, the PHICH is a channel that delivers DL ACK/NACK information for ULHARQ. The PHICH includes one REG and is scrambled cell-specifically. AnACK/NACK is indicated in one bit and modulated in Binary Phase ShiftKeying (BPSK). The modulated ACK/NACK is spread with a Spreading Factor(SF) of 2 or 4. A plurality of PHICHs mapped to the same resources forma PHICH group. The number of PHICHs multiplexed into a PHICH group isdetermined according to the number of spreading codes. A PHICH (group)is repeated three times to obtain a diversity gain in the frequencydomain and/or the time domain.

The PDCCH is a physical DL control channel allocated to the first n OFDMsymbols of a subframe. Herein, n is 1 or a larger integer indicated bythe PCFICH. The PDCCH occupies one or more CCEs. The PDCCH carriesresource allocation information about transport channels, PCH andDL-SCH, a UL scheduling grant, and HARQ information to each UE or UEgroup. The PCH and the DL-SCH are transmitted on a PDSCH. Therefore, aneNB and a UE transmit and receive data usually on the PDSCH, except forspecific control information or specific service data.

Information indicating one or more UEs to receive PDSCH data andinformation indicating how the UEs are supposed to receive and decodethe PDSCH data are delivered on a PDCCH. For example, on the assumptionthat the Cyclic Redundancy Check (CRC) of a specific PDCCH is masked byRadio Network Temporary Identity (RNTI) “A” and information about datatransmitted in radio resources (e.g. at a frequency position) “B” basedon transport format information (e.g. a transport block size, amodulation scheme, coding information, etc.) “C” is transmitted in aspecific subframe, a UE within a cell monitors, that is, blind-decodes aPDCCH using its RNTI information in a search space. If one or more UEshave RNTI “A”, these UEs receive the PDCCH and receive a PDSCH indicatedby “B” and “C” based on information of the received PDCCH.

FIG. 6 illustrates REs used for configuring a DL control channel in theLTE system. Specifically, FIG. 6(a) illustrates REs of a DL controlchannel in the case of 1 or 2 Tx antennas in an eNB and FIG. 6(b)illustrates REs of a DL control channel in the case of 4 Tx antennas inan eNB. Although a different RS pattern is used according to the numberof Tx antennas, REs are configured for a DL control channel in the samemanner.

Referring to FIG. 6, a basic resource unit of a DL control channel is anREG. The REG includes four contiguous REs except for REs carrying RSs.REGs are marked with bold lines in FIG. 6. A PCFICH and a PHICH include4 REGs and 3 REGs, respectively. A PDCCH is configured in units of aControl Channel Element (CCE), each CCE including 9 REGs.

To determine whether a PDCCH including L CCEs is transmitted to a UE,the UE is configured to monitor M^((L)) (≧L) CCEs that are arrangedcontiguously or in a predetermined rule. L that the UE should considerfor PDCCH reception may be a plural value. CCE sets that the UE shouldmonitor to receive a PDCCH are referred to as a search space. Forexample, the LTE system defines search spaces as illustrated in [Table1].

TABLE 1 Number of Search space S_(k) ^((L)) PDCCH Aggregation candidatesType level L Size [in CCEs] M^((L)) DCI formats UE- 1 6 6 0, 1, 1A, 2B,specific 2 12 6 1D, 2, 2A, 2B, 4 4 8 2 8 16 2 Common 4 16 4 0, 1A, 1C, 816 2 3/3A

In [Table 1], L is a CCE aggregation level, that is, the number of CCEsin a PDCCH, S_(k) ^((L)) is a search space with CCE aggregation level L,and M^((L)) is the number of candidate PDCCHs to be monitored in thesearch space with CCE aggregation level L.

Search spaces are classified into a UE-specific search space accessibleonly to a specific UE and a common search space accessible to all UEswithin a cell. A UE monitors common search spaces with CCE aggregationlevels 4 and 8 and UE-specific search spaces with CCE aggregation levels1, 2, 4, and 8. A common search space and a UE-specific search space mayoverlap with each other.

For each CCE aggregation level, the position of the first CCE (a CCEhaving the smallest index) of a PDCCH search space allocated to a UEchanges in every subframe. This is called PDCCH search space hashing.

A CCE may be distributed across a system band. More specifically, aplurality of logically contiguous CCEs may be input to an interleaverand the interleaver may permute the sequence of the input CCEs on an REGbasis. Accordingly, the time/frequency resources of one CCE aredistributed physically across the total time/frequency area of thecontrol region of a subframe. As a control channel is configured inunits of a CCE but interleaved in units of an REG, a frequency diversitygain and an interference randomization gain may be maximized.

FIG. 7 illustrates a structure of a UL subframe in the LTE system.

Referring to FIG. 7, a UL subframe may be divided into a control regionand a data region. A Physical Uplink Control Channel (PUCCH) includingUplink Control Information (UCI) is allocated to the control region anda Physical uplink Shared Channel (PUSCH) including user data isallocated to the data region. The middle of the subframe is allocated tothe PUSCH, while both sides of the data region in the frequency domainare allocated to the PUCCH. Control information transmitted on the PUCCHmay include an HARQ ACK/NACK, a CQI representing a downlink channelstate, an RI for MIMO, a Scheduling Request (SR) requesting UL resourceallocation. A PUCCH for one UE occupies one RB in each slot of asubframe. That is, the two RBs allocated to the PUCCH arefrequency-hopped over the slot boundary of the subframe. Particularly,PUCCHs with m=0, m=1, m=2, and m=3 are allocated to a subframe in FIG.6.

Due to emergence and proliferation of various devices requiringMachine-to-Machine (M2M) communication and a large amount of data, theamount of required data over a cellular network is increasing very fastin a current wireless communication environment. To satisfy the highdata amount requirement, communication technology is being developed tocarrier aggregation that enables efficient use of more frequency bands,MIMO that increases a data capacity in a limited frequency, CoordinatedMulti-Point (CoMP), etc. Furthermore, the communication environment isevolving toward highly populated nodes accessible to users. A systemhaving highly populated nodes may increase system performance throughcooperation between nodes. This technology has very excellentperformance, relative to a non-cooperative case where each node servesas an independent Base Station (BS), Advanced BS (ABS), Node B, eNB,Access Point (AP), or the like.

FIG. 8 illustrates a configuration of a multi-node system as afuture-generation communication system.

Referring to FIG. 8, if all nodes collectively operate as an antenna setof a cell, with their transmission and reception under control of acontroller, this system may be regarded as a Distributed Multi-NodeSystem (DMNS) forming one cell. The individual nodes may be allocatednode IDs or may operate as antennas of the cell without node IDs.However, if the nodes have different cell IDs, this system may beregarded as a multi-cellular system. If multiple cells are overlaidaccording to their coverage, this is called a multi-tier network.

Meanwhile, a node may be any of a Node B, an eNB, a Picocell eNB (PeNB),a Home eNB (HeNB), a Remote Radio Head (RRH), a relay, a distributedantenna, etc. At least one antenna is installed in one node. A node isalso called a transmission point. While a node refers to an antennagroup with antennas apart from each other by a predetermined distance orfarther, the present invention may be implemented even though a node isdefined as an antenna group irrespective of the distance betweenantennas.

Owing to the introduction of the afore-described multi-node system andrelay nodes, various communication techniques have become available,thereby improving channel quality. However, to apply MIMO and inter-cellcooperative communication technology to a multi-node environment, a newcontrol channel is required. In this context, Enhanced PDCCH (E-PDCCH)is under discussion and it is regulated that the E-PDCCH is allocated toa data region (hereinafter, referred to as a PDSCH region) other than alegacy control region (hereinafter, referred to as a PDCCH region).Since the E-PDCCH enables transmission of control information about anode to each UE, shortage of the legacy PDCCH region may be overcome.The E-PDCCH may be accessible only to LTE-A UEs, not to legacy UEs.

FIG. 9 illustrates an example of an E-PDCCH and a PDSCH scheduled by theE-PDCCH.

Referring to FIG. 9, an E-PDCCH may occupy a part of a PDSCH region thattypically carries data. A UE should perform blind decoding to determinethe presence or absence of an E-PDCCH directed to the UE. The E-PDCCHfunctions to schedule (i.e. PDSCH and PUSCH control) like a legacyPDCCH. However, if more UEs are connected to nodes such as RRHs and thusmore E-PDCCHs are allocated to the PDSCH region, the UE should performmore blind decodings, thus experiencing increased complexity.

The present invention proposes a method for effectively mappingresources to an E-PDCCH that is a control channel transmitted in alegacy data region, instead of a legacy PDCCH.

Embodiment 1

If an E-PDCCH is modulated in a modulation scheme having a highmodulation order such as 16-ary Quadrature Amplitude Modulation (16QAM)or transmitted in multiple layers, the E-PDCCH may be transmitted moreeffectively. In general, a control channel is modulated in a modulationscheme having a low modulation order such as Quadrature Phase ShiftKeying (QPSK), for reception stability, and limited to one transmissionlayer, for reduction of interference in spatial resources. However, if aUE is placed in a very good channel state, it is preferable to increasethe transmission rate of a control channel by means of a high modulationorder or multi-layer transmission. If a control channel E-PDCCH ismodulated with a high modulation order or transmitted in multiplelayers, it is preferred that a data channel PDSCH is also modulated witha high modulation order or transmitted in multiple layers.

Accordingly, the present invention proposes that a data channel ismodulated with a modulation order equal to or higher than a controlchannel and/or transmitted in as many layers as or more layers than thecontrol channel, based on the above-described relationship between thecontrol channel and the data channel.

According to the present invention, some field carrying information onthe control channel is meaningless and the control channel is receivedeffectively by utilizing the field. For example, the control channeltypically includes a field indicating a Modulation and Coding Scheme(MCS) used for the data channel. When the control channel is modulatedin 16QAM, the data channel is not modulated in QPSK in the presentinvention. Therefore, the number of bits in the MCS field may be reducedby excluding a value indicating QPSK and thus a coding rate may beincreased in spite of the same number of REs. Or if a UE assumes non-useof QPSK although the number of bits in the MCS field is maintained, theprobability of demodulation based on the MCS field indicating QPSK canbe ignored. In this case, the probability of succeeding in demodulatingthe control channel can be increased.

If the control channel is transmitted using a modulation scheme having ahigh modulation order or multiple layers as described above, intendedinformation may be transmitted with a smaller amount of resources.Therefore, the present invention proposes that a basic unit for E-PDCCHaggregation levels is configured with fewer REs for an E-PDCCHtransmitted using a high-order modulation scheme or multiple layers thanfor an E-PDCCH transmitted using a low-order modulation scheme or asingle layer.

For example, an E-PDCCH modulated in QPSK is transmitted, using one PRBas a basic aggregation unit (e.g., an Enhanced CCE (E-CCE)). That is,E-PDCCHs with aggregation levels 1, 2, 4, and 8 are transmitted in 1, 2,4, and 8 PRBs, respectively.

On the other hand, for an E-PDCCH modulated in 16QAM, the REs of one PRBare divided into two subsets and a basic aggregation unit (e.g. E-CCE)is configured with one of the subsets. Herein, the subset is referred toas a sub-PRB. That is, E-PDCCHs with aggregation levels 1, 2, 4, and 8are transmitted in 1, 2, 4, and 8 sub-PRBs, respectively. An E-PDCCHconfigured with two sub-PRBs may be transmitted in two sub-PRBs of thesame PRB, that is, in one PRB, or in two sub-PRBs of two PRBs apart fromeach other in the frequency domain, each sub-PRB from one PRB to achievea frequency diversity gain.

Or it may be contemplated that the size of an E-CCE is fixed, forexample, to one sub-PRB and a plurality of E-CCEs are aggregated as abasic unit for configuring an E-PDCCH. In the above embodiment of thepresent invention, one E-CCE corresponding to one sub-PRB is consideredas a basic unit for an E-PDCCH modulated in 16QAM, whereas two E-CCEscorresponding to two sub-PRBs is considered as a basic unit for anE-PDCCH modulated in QPSK.

FIG. 10 illustrates an example of defining a basic unit for configuringan E-PDCCH as two sub-PRBs according to an embodiment of the presentinvention.

Referring to FIG. 10, a PRB in the first slot of a subframe is dividedinto two subsets, subset A and subset B and the two subsets, subset Aand subset B are defined as a basic unit for an E-PDCCH. It is notedfrom FIG. 10 that subset A and subset B are mapped in a frequency-firstmanner.

FIG. 10 is purely exemplary. Thus, it is to be clearly understood thatone PRB may be divided into two or more sub-PRBs in many other ways.

In another example of the present invention, it is possible todifferentiate a modulation scheme or the number of transmission layersin one search space, for each aggregation level. For example, channelstate may be good generally at a low aggregation level. Therefore, useof a high-order modulation scheme or multi-layer transmission may bepreferable for the low aggregation level. On the contrary, a highaggregation level is used in a poor channel state or when a transmitterdoes not know an accurate channel state. Thus, a low-order modulationscheme or single-layer transmission is preferable for the highaggregation level, for a more stable operation.

For example, 16QAM may be used for aggregation level 1 and QPSK may beused for aggregation levels 2, 4, and 8. In another example, while anE-PDCCH with aggregation level 1 is transmitted in two layers, anE-PDCCH with aggregation level 2, 4, or 8 is transmitted in one layer.

In addition, a relationship between a basic aggregation unit (E-CCE) andthe afore-described modulation scheme and number of transmission layersmay be utilized. That is, resources are aggregated for an E-PDCCH usinga small set of REs as a basic unit for an aggregation level using ahigh-order modulation or multi-layer transmission, whereas resources areaggregated for an E-PDCCH using a large set of REs as a basic unit foran aggregation level using a low-order modulation or single-layertransmission.

For example, an E-PDCCH is transmitted in units of a sub-PRB ataggregation level 1 using 16QAM, whereas an E-PDCCH is transmitted inunits of a PRB at aggregation level 2, 4, or 8 using QPSK. With thisoperation, the number of bits in a single E-PDCCH may be maintained tobe a multiple of a basic unit, K bits because n×K bits are transmittedat aggregation level n even though a modulation scheme or the number oftransmission layers is changed (K is the number of bits of DL controlinformation at aggregation level 1).

For example, if an E-PDCCH with aggregation level 1 is modulated in16QAM and transmitted in one sub-PRB occupying a half of the REs of aPRB, the E-PDCCH delivers K bits in total. In the case of aggregationlevel 2, 4, or 8, an E-PDCCH is modulated in QPSK, thus decreasing thenumber of transmission bits per RE to a half. However, since 2, 4, or 8PRBs are used at aggregation level 2, 4, or 8, the number of used REs isincreased by 4, 8, or 16 times from at aggregation level 1. As aconsequence, the E-PDCCH delivers 2K, 4K, or 8K bits. It is alsopossible to transmit an E-PDCCH with aggregation level 1 or 2 in unitsof a sub-PRB by use of 16QAM and an E-PDCCH with aggregation level 4 or8 in units of a PRB by use of QPSK.

To implement the above-described operations, an eNB may indicate anaggregation level and a modulation scheme/the number of transmissionlayer/a basic aggregation unit (E-CCE) to be used at the aggregationlevel to a UE by high-layer signaling such as RRC signaling.

In another method for reducing the number of REs in a basic aggregationunit for an E-PDCCH, the number of OFDM symbols used in transmitting theE-PDCCH may be reduced. For example, an E-PDCCH using QPSK istransmitted in one slot of a subframe (i.e. 4^(th) to 7^(th) OFDMsymbols of the subframe) as illustrated in FIG. 10, whereas an E-PDCCHusing 16QAM is transmitted in fewer OFDM symbols (e.g., 4^(th) and5^(th) OFDM symbols). Since the number of symbols for an E-PDCCH isreduced, E-PDCCH transmission may be completed early, particularly in ahigh-order modulation scheme and more data may be transmitted in thefollowing symbols.

Embodiment 2

Aside from the modulation order or the number of transmission layers ofan E-PDCCH, the size of a basic aggregation unit for the E-PDCCH may beadjusted according to other communication configurations. For example,the size of the basic aggregation unit may be adjusted according to theposition of the starting or ending symbol of the E-PDCCH, which maydefine the amount of resources available to the E-PDCCH.

First of all, a system bandwidth may be considered. The E-PDCCH carriesinformation about frequency resources occupied by scheduled data in afrequency area configured by a current cell. In general, the number ofbits in the frequency resource information is proportional to the systembandwidth. If the system bandwidth is narrow, the number of bitstransmitted in the E-PDCCH is also decreased. When the number of bits inthe E-PDCCH is decreased to or below a predetermined value, reduction ofthe size of the basic aggregation level for the E-PDCCH or reduction ofthe number of symbols used for transmission of the E-PDCCH may be moreeffective in resource utilization.

Second, the number of CRS antenna ports or a PDCCH length may beconsidered. An eNB should transmit a CRS and a PDCCH at least in somefirst symbols of a subframe to support legacy UEs that measure channelsand receive a control signal using the CRS and the PDCCH. If the eNBdoes not transmit a CRS or a PDCCH in a specific subframe (or transmitsa CRS or a PDCCH in a limited number of symbols, for example, in onesymbol), E-PDCCH transmission may be completed early in the subframe.Therefore, it is more effective to use the following symbols in datatransmission.

Now, a description will be given of a method for adjusting a basicaggregation unit for an E-PDCCH, for example, according to a PDCCHlength (or the index of the starting symbol of the E-PDCCH). The E-PDCCHis transmitted preferably after PDCCH transmission. The number ofsymbols occupied by a PDCCH may vary with a subframe configuration andas a result, the starting symbol of the E-PDCCH may also be changedaccording to the subframe configuration.

If the E-PDCCH starts early (for example, in symbol #0 or symbol #1),relatively many REs of a single PRB may be used for transmission of theE-PDCCH. Accordingly, the basic E-PDCCH aggregation unit is preferablyset to a sub-PRB smaller than a PRB. On the contrary, if the E-PDCCHstart later (for example, in symbol #2 or symbol #3), a relatively smallnumber of REs in a single PRB are available. Then, there is no need fornecessarily aggregating resources for the E-PDCCH on a sub-PRB basis.Rather, the aggregation is preferably performed on a PRB basis.

FIG. 11 illustrates an example of defining a basic aggregation unit foran E-PDCCH according to another embodiment of the present invention.Particularly, a basic aggregation unit is changed according to thestarting symbol of an E-PDCCH on the assumption of aggregation level 2in FIG. 11.

Referring to FIG. 11, (a) indicates a case where when an E-PDCCH startslate (for example, in symbol #2 or symbol #3), PRB-wise aggregation isperformed for the E-PDCCH. (b) indicates a case where when an E-PDCCHstarts early (for example, in symbol #0 or symbol #2), relatively manyREs of a single PRB are available for E-PDCCH transmission and thus abasic aggregation unit is set to a sub-PRB smaller than a PRB for theE-PDCCH.

While the sub-PRB-wise aggregation scheme and the PRB-wise aggregationscheme have been described above, the present invention is not limitedthereto. Rather, the present invention covers a more generalizedsituation as described below. That is, sub-PRB type n is defined bydividing one PRB into n subsets and n is changed according to thestarting symbol of E-PDCCH transmission. Therefore, if an E-PDCCH startsearly, a larger n value is used because more REs of a single PRB areavailable to the E-PDCCH (i.e. a single PRB is divided into moresub-PRBs and the resulting sub-PRB type is used as a basic aggregationunit). In contrast, if an E-PDCCH starts large, a smaller n value isused (i.e. a single PRB is divided into fewer sub-PRBs and the resultingsub-PRB type is used as a basic aggregation unit).

If a UE is aware that an E-PDCCH starts early, the UE monitors a searchspace using a larger n value than during blind decoding of an E-PDCCH(i.e., using a basic aggregation unit acquired by dividing a single PRBinto more sub-PRBs). On the contrary, if the UE is aware that an E-PDCCHstarts late, the UE monitors a search space using a smaller n value(i.e., using a basic aggregation unit acquired by dividing a single PRBinto fewer sub-PRBs).

Further, n may be changed according to the ending symbol of an E-PDCCH.When the E-PDCCH ends relatively late, a larger n value may be used(i.e., a basic aggregation unit acquired by dividing a single PRB intomore sub-PRBs may be used) because more REs of a single PRB areavailable to the E-PDCCH. On the other hand, when the E-PDCCH endsrelatively early, a smaller n value may be used (i.e., a basicaggregation unit acquired by dividing a single PRB into fewer sub-PRBsmay be used).

Likewise, if a UE is aware that an E-PDCCH ends relatively late, the UEmonitors a search space using a larger n value (i.e., using a basicaggregation unit acquired by dividing a single PRB into more sub-PRBs),whereas if the UE is aware that an E-PDCCH ends relatively early, the UEmonitors a search space using a smaller n value (i.e., using a basicaggregation unit acquired by dividing a single PRB into fewer sub-PRBs).

Embodiment 3

Without associating information such as the number of REs in a basicaggregation unit for an E-PDCCH or the position of the starting/endingsymbol of the E-PDCCH with the above-described E-PDCCH modulation schemeor system bandwidth, an eNB may freely set the information and signalthe information in a specific message by high-layer signaling such asRRC signaling. The starting/ending time of the E-PDCCH may be differentin each subframe.

For example, since a legacy UE searches for its PDCCH, assuming that thePDCCH is always transmitted in two symbols in an MBSFN subframe of acell for which 4-antenna port CRSs are configured, it may be impossibleto use the first or second symbol of the subframe for E-PDCCHtransmission. Accordingly, a third embodiment of the present inventionproposes that the position of the starting/ending symbol of an E-PDCCHis set to be different in each subframe. One thing important is that theposition of the starting/ending symbol of an E-PDCCH is differentirrespective of transmission or non-transmission of a PDCCH.

More specifically, an eNB transmits a subframe pattern and indicates useof a specific starting/ending point of an E-PDCCH in subframes indicatedby the subframe pattern. Then the eNB transmits another subframe patternand indicates use of a specific starting/ending point of an E-PDCCH inother subframes indicated by the subframe pattern. Without signaling thesubframe patterns, subframes may simply be divided into an MBSFNsubframe and a non-MBSFN subframe (i.e. a general subframe). In thiscase, it is possible to indicate the position of the starting/endingpoint of an E-PDCCH for each of the MBSFN subframe and the non-MBSFNsubframe. Once the starting OFDM symbol of an E-PDCCH is determined inthe above-described method, a PDSCH scheduled by the E-PDCCH preferablystarts in the same OFDM symbol.

FIG. 12 is a block diagram of a communication apparatus according to anembodiment of the present invention.

Referring to FIG. 12, a communication apparatus 1200 includes aprocessor 1210, a memory 1220, a Radio Frequency (RF) module 1230, adisplay module 1240, and a User Interface (UI) module 1250.

The communication device 1200 is shown as having the configurationillustrated in FIG. 12, for the convenience of description. Some modulesmay be added to or omitted from the communication apparatus 1200. Inaddition, a module of the communication apparatus 1200 may be dividedinto more modules. The processor 1210 is configured to performoperations according to the embodiments of the present inventiondescribed before with reference to the drawings. Specifically, fordetailed operations of the processor 1210, the descriptions of FIGS. 1to 11 may be referred to.

The memory 1220 is connected to the processor 1210 and stores anOperating System (OS), applications, program codes, data, etc. The RFmodule 1230, which is connected to the processor 1210, upconverts abaseband signal to an RF signal or downconverts an RF signal to abaseband signal. For this purpose, the RF module 1230 performsdigital-to-analog conversion, amplification, filtering, and frequencyupconversion or performs these processes reversely. The display module1240 is connected to the processor 1210 and displays various types ofinformation. The display module 1240 may be configured as, not limitedto, a known component such as a Liquid Crystal Display (LCD), a LightEmitting Diode (LED) display, and an Organic Light Emitting Diode (OLED)display. The UI module 1250 is connected to the processor 1210 and maybe configured with a combination of known user interfaces such as akeypad, a touch screen, etc.

The embodiments of the present invention described above arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim by asubsequent amendment after the application is filed.

The embodiments of the present invention may be achieved by variousmeans, for example, hardware, firmware, software, or a combinationthereof. In a hardware configuration, the methods according to exemplaryembodiments of the present invention may be achieved by one or moreApplication Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, microcontrollers, microprocessors,etc.

In a firmware or software configuration, an embodiment of the presentinvention may be implemented in the form of a module, a procedure, afunction, etc. Software code may be stored in a memory unit and executedby a processor. The memory unit is located at the interior or exteriorof the processor and may transmit and receive data to and from theprocessor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the method and apparatus for transmitting a downlink controlchannel at a BS in a wireless communication system have been describedmainly in the context of a 3GPP LTE system, they are applicable to manyother wireless communication systems.

What is claimed is:
 1. A method for transmitting an enhanced physicaldownlink control channel (EPDCCH) to a user equipment (UE) at a basestation (BS) in a wireless communication system, the method comprising;determining a number of enhanced control channel elements (ECCEs) thatform an aggregation unit for the EPDCCH, based on whether a downlinkbandwidth is equal to or larger than a first threshold value or whethera number of available resources for the EPDCCH is smaller than a secondthreshold value; and transmitting the EPDCCH using one or moreaggregation units to the UE.
 2. The method of claim 1, wherein each ofthe one or more aggregation units is configured with the determinednumber of ECCEs.
 3. The method of claim 1, wherein a number ofaggregation units used for transmitting the EPDCCH is determined basedon an aggregation level of the downlink control channel.
 4. The methodof claim 1, wherein: if the downlink bandwidth is equal to or largerthan the first threshold value or if the number of available resourcesfor the EPDCCH is smaller than the second threshold value, the number ofthe ECCEs is determined to be a value larger than 1; and otherwise, thenumber of the ECCEs is determined to be
 1. 5. The method of claim 1,wherein the number of available resources for the EPDCCH is determinedaccording to a number of resource elements for reference signals and anindex of starting symbol of the EPDCCH.
 6. A base station (BS) in awireless communication system, the BS comprising: a processor configuredto determine a number of enhanced control channel elements (ECCEs) thatform an aggregation unit for an enhanced physical downlink controlchannel (EPDCCH), based on whether a downlink bandwidth is equal to orlarger than a first threshold value or whether a number of availableresources for the EPDCCH is smaller than a second threshold value; and aRadio Frequency module configured to transmit the EPDCCH using one ormore aggregation units to a user equipment (UE).
 7. The BS of claim 6,wherein each of the one or more aggregation units is configured with thedetermined number of ECCEs.
 8. The BS of claim 6, wherein a number ofaggregation units used for transmitting the EPDCCH is determined basedon an aggregation level of the downlink control channel.
 9. The BS ofclaim 6, wherein: if the downlink bandwidth is equal to or larger thanthe first threshold value or if the number of available resources forthe EPDCCH is smaller than the second threshold value, the processordetermines the number of the ECCEs to be a value larger than 1; andotherwise, the processor determines the number of the ECCEs to be
 1. 10.The BS of claim 6, wherein the number of available resources for theEPDCCH is determined according to a number of resource elements forreference signals and an index of starting symbol of the EPDCCH.